Most electronic circuits are now implemented within microelectronic circuits, commonly referred to as integrated circuits or “chips”. Such a chip comprises a semiconductor die carrying the microelectronic circuit encapsulated within a plastics or ceramic case. This enables the chip to be bonded or soldered to circuit boards and the like for the formation into more complex products. Many applications of microelectronic circuitry may require communication of signals between a relatively low voltage domain, where for example the supply rails may differ from each other by only a few volts, and a higher voltage domain containing higher voltage components as might be found in the energy, signaling, automation, communications or motor control arenas. This list is not exhaustive. There are also safety critical applications such as medical applications, where high voltages must not be allowed to propagate from the circuit towards a patient being monitored. Although these high voltages may not be generated deliberately, they might occur in certain fault modes, for example if a power supply were to develop a fault, or if a lightning strike were to induce overvoltage transients into power supply or data connections to the device.
It is known to isolate the low voltage domain and the high voltage domain of a circuit or system from one-another using “isolators”. These have typically involved discrete components, such a signal transformers, being mounted on a circuit board between a low voltage side of the board and the high voltage side of the board. More recently “chip scale” isolators have become available. Within a “chip scale” isolator components that communicate between the low voltage and high voltage sides or domains of the circuit are provided within a package of the type known in the provision of integrated circuits, such as a dual in line package. Such a package may contain two or three dies which are co-packaged such that the device still presents itself to a user as an integrated circuit.
The reduced dimensions in chip scale isolators start to give rise to breakdown mechanisms not seen in non-chip scale isolators, i.e. discrete component isolators. Isolators are often given a rating called the breakdown voltage. When the voltage difference between the low and high voltage parts of the isolator exceeds the breakdown voltage, the dielectric material between the parts may exhibit electrical breakdown and become electrically conductive, no longer performing as an effective insulator. To increase the breakdown voltage, an isolator can be manufactured with a thicker layer of dielectric material between the electrodes.